A process has been known in which development is carried out in such a way that two or more color toners are developed upon one another directly on the surface of an electrophotographic photoreceptor by using developers for electrostatic photography to form a color image, and the color image is immediately transferred onto a receiving material such as printing paper to obtain a color image print, a color image copy or a color proof (proof sheet for printing).
The above-described development process includes dry development process and wet development process. The wet development process is preferred because it provides color images which do not suffer from color drift and have high resolution in comparison with color images obtained by the dry development process. However, it is very difficult that wet toner images are completely transferred from the surface of the photoreceptor directly onto paper.
JP-A-2-272469 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein after a non-aqueous solvent is fed between the photoreceptor and the receiving material during transfer, color images are electrostatically transferred to solve the above-described problem.
Further, JP-A-2-115865 and JP-A-2-115866 disclose a method wherein after a transparent film is previously laminated onto the surface of the photoreceptor, a wet toner image is formed on the film by an electrophotographic process, and the film is released from the photoreceptor and laminated on plain paper. There is described that it is preferred that the film to be laminated has a thickness of 9 .mu.m. However, the production and handling of the film having such a thickness as described above is difficult and an alternative method is demanded.
Furthermore, JP-B-2-43185 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method wherein exposure to light is made from the back side of a transparent electrophotographic photoreceptor to form an overlapped color separation image on a dielectric support, and the image together with the support is transferred onto a receiving material. In this method, exposure to light is made from the transparent support side of the photoreceptor, and further the electrically conductive layer must be transparent. Accordingly, this method is disadvantageous in cost.
JP-A-1-112264, JP-A-1-281464 and JP-A-3-11347 propose that in an electrophotographic transfer process using the dry development process, a releasable transfer layer is previously provided on the surface of the photoreceptor, a toner image is formed thereon, and the toner image together with the transfer layer is transferred onto paper.
However, when the photoreceptor is repeatedly used in this method, there are disadvantages that an extra operation is required during operation, and it is difficult to form the transfer layer. Further, when the photoreceptor provided with the transfer layer (or a releasing layer) is used, the photoreceptor must be disposable. Accordingly, this method is disadvantageous in cost.
JP-A-2-264280 discloses a method wherein a toner image on the photoreceptive layer is transferred onto a primary intermediate transfer medium having high smoothness and then transferred onto a final receiving material. Further, JP-A-3-243973 and JP-A-4-9087 propose a method wherein a final color image of good quality is obtained by using a special transfer medium even when wet toner system is used.
In the above-described methods, it is conventionally considered that a toner image is clearly transferred without being affected by irregularities on the surfaces of the receptors. However, since the toner image is directly transferred onto a primary intermediate transfer medium and then onto the final receiving material, the resulting color image is deteriorated, for example, a lack of the toner image or unevenness in image density is found, particularly a lack of fine images such as fine lines, fine letters, etc. is found.
Further, toner image is left behind on the surface of the photoreceptor after the toner image is transferred. This causes problems in that when the photoreceptor is repeatedly used, the surface of the photoreceptor must be cleaned, a cleaning device must be provided and the surface of the photoreceptor is damaged by cleaning.
As described above, conventional color image forming method using an intermediate transfer medium has problems in that a fully satisfying color image can not be obtained and that when the intermediate medium is repeatedly used, the effect of the intermediate medium is changed, it is difficult to keep a stable performance over a long period of time, disposable members must be used to keep its performance, and a special transfer medium is required.
The present invention is intended to solve the problems associated with conventional image forming methods in electrophotographic process.